Micromachining of silicon wafers for the fabrication of microelectronics sensors is well known in the semiconductor arts. Generally, bulk micromachining processes used in the manufacture of silicon sensors involve forming a semiconductor sensing structure in a silicon wafer by wet chemical etching the bulk silicon at a surface of the wafer. Forming a sensing structure, such as a beam or diaphragm, directly from silicon is advantageous due to the excellent mechanical properties of silicon. In addition, micromachining techniques are compatible with batch fabrication methods employed with semiconductor devices, yielding a relative low cost, reliable and accurate silicon sensor that is suitable for use in many applications. Bulk micromachined sensing structures generally exhibit superior electrical properties, in that the etching process does not induce stresses that would adversely affect the accuracy of the sensor's output.
The silicon wafer in which the sensing structure is formed must be processed to enable movement of the sensing structure to be detected, such as by piezoresistive or capacitive sensing elements. In addition, the silicon wafer often includes circuitry for conditioning the output signal of the sensing elements. The conditioning circuitry can be formed before or after the sensing structure is micromachined. However, it is generally preferable to minimize processing of the wafer after etching of the micromachined structure due to the fragility of the wafer and micromachined structure, the latter of which is often a cantilever, diaphragm or beam whose thickness is less than about ten micrometers. However, if micromachining is performed after formation of the circuitry, metallizatlon of the circuitry must be protected from the caustic wet chemical etchant, which is typically an anisotropic etchant such as potassium hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH). For this purpose, metallizations are typically shielded from the etchant by mechanical fixtures, spin-on films or deposited films. However, each has drawbacks.
Spin-on films, such as waxes, photoresists and polyimides, cannot withstand caustic, high temperature conditions for extended periods. As such, these films are inadequate for use when performing a through-wafer etch.
Mechanical fixtures are usually constructed with o-rings for sealing the surface of the wafer from the etchant. These fixtures must be individually assembled on the production floor. If a dirt particle or o-ring defect is present, the wafer is typically ruined. In addition, because the o-ring must be compressed to generate an adequate seal, mechanical fixtures tend to induce stresses in the silicon wafer, particular in view of the relatively fragile state of the wafer.
Deposited films, such as oxides and nitrides, are often used to protect metallization during etching processes. However, prior art deposited films exhibit limited step coverage, i.e., the ability to simultaneously cover two or more surface features having different elevations on the surface of the wafer. Coverage of metallization with a single layer of film is generally limited to metal layers that are inadequate for high yield wire bonding. Under such circumstances, either a single thick layer or multiple layers of deposited film must be used. However, single deposited layers are prone to the formation of pin holes, while the deposition of several layers of film is expensive and the resulting multilayer film is prone to cracking. As a result, both approaches produce films whose ability to protect an underlying metallization is seriously limited. Finally, deposited films typically have relatively high etch rates in potassium hydroxide at elevated temperatures, and therefore pose a significant limitation to operating conditions of the etchant bath.
In general, the semiconductor industry is continuously seeking improvements over the above prior art methods for protecting metallizations during wet chemical etching of bulk silicon. This need is particularly in response to the desire to promote high volume manufacturing while increasing yield, improving etching conditions, and eliminating induced stresses.